1. Field of the Invention
The invention relates to an electrochemical cell which operates even at low temperatures and has a high durability.
2. Description of the Prior Art
Heretofore, electrochemical elements have been made by forming a sintered oxygen-ion-conductive solid electrolyte body by using for instance zirconium oxide added with calcia (CaO), yttria (Y.sub.2 O.sub.3), or the like and mounting electrodes such as platinum electrodes on the oxygen-ion-conductive solid electrolyte body. In fact, such electrochemical elements are electrochemical cells which have been used as oxygen sensors for detecting oxygen partial pressure in boiler exhaust gas and automobile exhaust gas or as oxygen pumping for pumping in oxygen to a gas or pumping out from a gas.
The electrode of the electrochemical cell for such use is required to have a small impedance at the boundary between it and the underlying body of oxygen-ion-conductive solid electrolyte, so as to generate an output electromotive force even at low temperatures. The electrode is also required to have a strong bond with the oxygen-ion-conductive solid electrolyte body, so as to provide a high durability even if it is used in a stream of very hot gas with a high flow rate.
The internal impedance of an electrochemical cell can be treated as the sum of the intrinsic impedance of the solid electrolyte body and the boundary impedance between the electrode and the underlying solid electrolyte body. The intrinsic impedance of the solid electrolyte is determined almost exclusively by the chemical composite thereof, while it is known that the boundary impedance varies over a wide range depending on the manner in which the electrode is attached to the solid electrolyte body. Thus, to produce an electrochemical cell which operates and generates a significant electromotive force at low temperatures, the boundary impedance at the electrode must be minimized.
In general, platinum group metals are used as the electrode of the electrochemical cell for such use, which are provided by chemical plating methods, physical plating methods, baking methods and the like on the surface of the solid electrolyte body. However, the electrode thus provided has a shortcoming in that the bond of the platinum group alloy with the underlying oxygen-ion-conductive solid electrolyte body is a mere mechanical coupling, and the electrode plate tends to float fairly easily from the surface of the solid electrolyte, when exposed to thermal stress or the like, resulting in an increase of the boundary impedance or even exfoliation of electrode in the worst case.
It is also known that an electrode with a low boundary impedance can be formed by applying a film of a platinum group metal on the surface of solid electrolyte by plating or baking, impregnating interstices in the thus applied film with a solution of an oxide forming metal compound, and heating the electrode so as to produce zirconium oxide and the like in the interstices as a decomposition product thereof. The thus formed electrode has a low boundary impedance, but is has a shortcoming in that its bond with the underlying oxygen-ion-conductive solid electrolyte is weak as in the case described above. In short, the boundary impedance is improved but the bonding strength is not improved.
If an improvement of the bonding strength of such electrode is tried by effecting the heating of the metal oxide solution at 1,100.degree. C. or higher while expecting possible reaction or sintering of the metal oxide with the underlying oxygen-ion-conductive solid electrolyte, the boundary impedance increases considerably. Thus, due to the restriction from the standpoint of boundary impedance, there are limits to improvement of the bonding strength of the electrode to the underlying solid electrolyte in this case too.
As an electrode of an electrochemical cell having a high bonding strength, a cermet electrode for the electrochemical cell such as an oxygen concentration cell has been known. The cermet electrode is formed by applying an electrode-forming mixture consisting of powder of a platinum group metal with powder of an oxygen-ion-conductive solid electrolyte on a desired surface of the underlying oxygen-ion-conductive solid electrolyte body, and firing them at a temperature high enough to result in sufficient reaction of the underlying solid electrolyte with the oxygen-ion-conductive electrolyte in the electrode-forming mixture for establishing a firm bond therebetween. However, such a cermet electrode for an oxygen concentration cell and the like has a shortcoming in that it has a high boundary impedance because it is treated at a high temperature sufficient for causing firm bond between the solid electrolyte in the electrode-forming mixture and the underlying oxygen-ion-conductive solid electrolyte body, and hence it does not produce any significant electromotive force at low temperatures.